TEMPERATURE DEPENDENT SPIN CORRELATIONS IN AN ORGANIC MAGNET : [DMTzNC]2-[TCNQ]3

HAL Id: jpa-00228909

https://hal.archives-ouvertes.fr/jpa-00228909

Submitted on 1 Jan 1988

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

TEMPERATURE DEPENDENT SPIN

CORRELATIONS IN AN ORGANIC MAGNET :

[DMTzNC]2-[TCNQ]3

S. Takagi, K. Nakatsu, M. Date

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, Supplbment au no 12, Tome 49, dkembre 1988

TEMPERATURE DEPENDENT SPIN CORRELATIONS IN AN ORGANIC

MAGNET: [DMTzNC]~

-

[TCNQ13

S. Takagi ( I ) , K. Nakatsu (,) and M. Date (,)

( I ) Faculty of Engineering, Kyushu Institute of Technology, Fukuoka 804, Japan

( 2 ) Faculty of Science, Kwansei Gakuin University, Hyogo 6@2, Japan

(3) Faculty of Science, Osaka University, Osaka 560, Japan

Abstract. - A new organic ion-radial salt, [3,3'-dimethyl-2,2'-thiazolinocyanine]2-[TCNQ]3

,

is synthesized and the crys-

tallographic and magnetic properties are investigated. In the salt unpaired electrons form spin-pairs below about 200 K

while the spin-pairs decouple with increasing temperature and one-dimensional spin correlations appear above about

200 K.

7,7',8.8'-Tetracyanoquinodimenthane(TCNQ) is a diamagnetic planar molecule with strong acceptor properties. It forms many kinds of organic anion- radical salts combing with the other molecules or ions 111. Since the planar TCNQ molecule usually stacks face-to-face making anion columns separated each other by cation columns, the TCNQ salts are regarded as the examples of low-dimensional electron systems and show peculiar electronic properties [2].

In this paper we report on the crystallographic and magnetic properies of a newly synthesized TCNQ salt, [3,3'-dimethyl-2,2'-thia~olinocyanine]~-[TCNQ],

([DMTzNCJ,

-

[TCNQ], ), which is one of the TCNQ salts with cyanine-dyefDYE) as a cation [3]. This salt belongs to so called 2:3 salt and is the first example of the 2:3 salt in the DYETCNQ salts. The mag- netic properties of the salt are different from those of 1:2 salts such as MEQSeC-[TCNQ], [4]. In MEQSeC- [TCNQ],

,

a clear triplet state ESR of excited state is found a t low temperatures, and the one- and two- dimensional correlation appears with increasing tem- perature. On the contrary, no triplet state ESR is ob- served in [DMTzNC],

-

[TCNQ], at any temperatures. The chemical structure of the two elements of the salt are shown in figure 1. The salt, [DMTzNC], -[TCNQ],

,

was synthesized by the stan- dard method [5]. The single crystals were grown by very slow evaporation of an acetonitrile solution.

A single crystal X-ray analysis of the salt has been performed [3]. Crystals of the salt are triclinic with the space group P i and lattice constants a = 9.976(3), b = 16.129(5), c = 8.371(2)A1 cw = 101.80(3),

P

=

106.02(3), y = 77.02(3)', U = 1246.7(7)

A3

and Z = 2. The crystal structure viewed along c-axis is shown in figure 2. There are two DYE and three TCNQ molecules in a unit cell. From the charge neutral- ity condition, the unit cell should contain two un- paired electrons so that two electrons per three TCNQ molecules are expected. The planar TCNQ molecules are stacked face-to-face to form a column along the a-

Me Me.

(a) D M T ~ N C + (b) TCNQ

Fig. 1.

-

Chemical structure of (a) DMTZNC+ and (b) TCNQ.

Fig. 2. - Crystal structure viewed along the c-axis.

axis. The planar DYE cations, DMTZNC+, are also stacked face-to-face to form a dirneric column paral- lel to the TCNQ column. Both the b*- and c*-axes are defined to be perpendicular to the ac- and ab*- planes, respectively. The angular dependence of the ESR linewidth was measured in both the b*- and c*- planes.

The temperature dependence of the spin- susceptibility was measured on the powder specimen of the salt in the temperature range of 77-340 K using Faraday-type magnetic balance [3]. The susceptibility is very small below 100 K. However, it increases mono- tonically with increasing temperature above 100 K and is 3.1

x

emu/mole at 300 K. The susceptibility below about 200 K can be explained by the spin-pair model with the exchange energy of about 500 K. Above 200 K, however, the observed points deviate from the model.

A single ESR absorption line with the g-value of 2.002 is observed for a single crystal in the tempera-

C8

-

1474 JOURNAL DE PHYSIQUE

ture range of 77-340 K [3]. The linewidth of the ESR absorption, AH, strongly depends on the temperature and on the direction of external magnetic field. The main origin of the linewidth seems to be the exchange- narrowed dipole width. The linewidth for a

//

H is

18 Oe a t 200 K and decreases t o 1.5 Oe at 295 K. The angular dependence in the b*-plane of the ESR linewidth above 220 K is shown in figure 3, where 8 is the angle between the a-axis and the direction of the external magnetic field. Similar angular depen- dence of the ESR linewidth is observed in the c*-plane. Such typical W-shaped angular dependences have been observed in the well-known one-dimensional magnet TMMC, of which spin dynamics is treated by the one- dimensional spin-diffusion theory 161.

The angular dependence of the ESR linewidth,

A H (8)

,

in TMMC is written as

where 6 is the angle between the chain axis and the direction of external magnetic field. On the contrary, the obversed linewidth shown in figure 3 is explained by

A H (6) = A ( 3 cos2 8 - lln

,

₍₂₎

where the index n is temperature dependent and

changes from 1.9 at 220 K t o 1.5 at 340 K as tem- perature increases. The ESR lineshape at 280 K for

a

//

H or A I H deviates from the Lorentzian line-

shape, though the deviation is not enough compared to that of TMMC [3]. These facts suggest that the one-dimensionality in the salt is not so ideal as that in TMMC but is better a t higher temperatures.

Fig. 3. - Angular dependence of the ESR linewidth in the b*-plane.

( 1 ) 2 : 3 Salt (a) Ground State

(b) Transfer State

(11) 1 : 2 Salt ( a ) Ground State

(b) Excited Triplet S_tgte--

( c ) Transfer State

Fig. 4. - Schematic spin models for the 2:3 and 112 salts.

Schematic spin models a t low temperatures are shown in figure 4. The spin-pair i n the 1:2 salt is well separated from the nearest neighbor pairs by two TCNQ molecules while only one TCNQ molecule ex- ists between pairs in the 2:3 salt 13, 41. Therefore, the spin-pair in the 1:2 salt shows isolateld properties such as well defined localized excited triplet state which is however masked by the electron transfer in the 2:3 salt. The spin-pairs in the 2:3 salt, [DMTzNC],

-

[TCNQ],

,

decouple through the electron transfer with increasing temperature and the one-dimensional spin correlation appears above about 200 K.

[I] Melby, L. R., Harder, R. J., Hertler, W. R., Mahler, W., Benson, R. E. and Mochel, W. E.,

J. Am. Chem. Soc. 84 (1962) 3374.

[2] Miller, J. S., Extended Linear Chain Compounds, Vol. 3 (Plenum Press. New York) 1983.

[3] Takagi, S., Nakatsu, K. and Date, M., J. Phys.

Soc. Jpn 57 (1988) 2154.

[4] Uemura, T., Takagi, S., Okuda, K. and Date, M., J. Phys. Soc. Jpn 51 (1982) 760.

[5] Klanderman, B. H. and HoestJerey, D. C., J. Chem. Phys. 51 (1969) 377.

[6] Dietz, R. E., Meriitt, F. R., Dingle, R., Hone, D., Silbernagel, B. G. and Richards, P. M., Phys.